The present invention relates to a digitally implemented demodulator for maritime and mobile data communications, and in particular to a fast frequency estimator which operates without using an acquisition preamble.
A typical packet format used for burst communications and signaling in a time division multiple access (TDMA) network employs a structure that includes an acquisition preamble at the start of the packet, followed by a known unique word (UW) pattern. The data portion of the packet then follows, with additional framing bits inserted periodically for long packets, with the packet ending in another known "end-of-packet" sequence.
The acquisition preamble that precedes the data portion of the packet typically incorporates an unmodulated carrier sequence for the carrier frequency and phase estimate at the receiver, followed by a clock recovery sequence for proper receiver clock phase alignment. The UW pattern is used for phase ambiguity resolution and burst time synchronization. After the preamble and UW segments have been received and acted upon, the receiver is ready to demodulate the ensuing data segment with the correct frequency, phase, and clock adjustments.
The unmodulated carrier segment appears as a single tone in the frequency spectrum for a short duration. The detection of the carrier can therefore be accomplished by frequency domain acquisition algorithms.
In an analog implementation, a frequency lock loop (FLL) may be used. A bank of analog bandpass filters followed by energy detectors may be used to give a coarse estimate of carrier frequency. A similar technique utilizes a bank of correlators tuned to several discrete carrier frequencies. In a digitally implemented receiver, a DFT/FFT-based algorithm may be used. Time domain techniques may also be used for the carrier frequency estimate, whereby the unmodulated carrier phase differences are computed periodically and an average estimate is obtained that is indicative of the rate of the phase change or the frequency offset.
The methods used for carrier frequency acquisition described above have limitations that preclude their use for communication networks that operate under hostile channel environments that include low signal-to-noise levels, large frequency offsets, Doppler frequency shifts, and multipath fading. The FLL technique, in particular, requires a finite "lock-in" time for the loop to acquire. The tracking range is also very limited if a high-resolution measurement is required. For data packets with very short acquisition preambles, or for preambleless packets, the FLL technique is disadvantageous. The FLL method may still be used during the data portion of the packet if the signal is squared to remove the modulation (for binary phase shift keying, or BPSK) but with a corresponding loss of operating signal-to-noise ratio that will compromise its operation for low channel signal-to-noise ratios.
The bandpass filter and correlator approaches become excessively complex if a very high resolution measurement is required. If the filters or correlators are of the analog type, they are also subject to drift and require precise components and frequent adjustments. In addition, signal detection is sensitive to the input signal level unless an automatic gain control (AGC) circuit is employed.
The straightforward FFT technique suffers the disadvantage of susceptibility to multipath that will affect the signal strengths in the frequency bins. A single DFT or FFT must also be very large (1024 points or greater) to operate successfully at very low values of E.sub.S / N.sub.O. A threshold to detect the signal presence will be unreliable unless external AGC is employed that will track the multipath fading. A tradeoff DFT/FFT computation time versus frequency resolution is also necessary. Fine frequency resolution translates to longer FFT computation time, and vice versa. In addition, this technique is suited for the case of a packet with an acquisition preamble. For preambleless packets, the straightforward FFT technique requires modifications.
The time domain frequency estimation technique is dependent on the unmodulated portion of the acquisition preamble, and is totally unsuited for preambleless packets. This method is also particularly sensitive to the magnitude of the frequency offset. If the frequency offset is large enough to cause a rotation of the signal vector through 2.pi. radians between the periodic estimates, then the measurement will be in error. In addition, due to the low operating signal-to-noise ratios, a long time average of the phase difference measurements are required, thus necessitating a long acquisition preamble.
The required preamble that precedes the data portion of the packet often constitutes an excessive overhead that reduces the channel transmission efficiencies for short data packets. Particularly in the case of mobile and maritime communication networks that experience signal fades of varying degrees, the system design mandates the use of very short data bursts (approximately a few hundred bits) that are relatively immune to fades. These bursts generally are used for the channel request and assignment functions from the remote terminal to a central location, typically a network coordination center. With additional channel impairments such as Doppler shifts, low carrier-to-noise ratios, and frequency offsets, long acquisition preambles usually are required to locate the carrier correctly, and to acquire its frequency and phase. The use of a long preamble in this case is an extremely undesirable overhead that seriously undermines the channel transmission efficiencies for a large communication network, with potentially thousands of such remote terminals.